This is more a curiosity... but apparently I have "unused config" nodes that are hidden in my Node-Red setup. And as of yet, I haven't figured out how to find out what they are.
And an additional 2 of them are apparently even more hidden then the rest
The Hidden nodes are those that don't match the filter - in other words, they are config nodes that are being used.
You have 37 'Global' config nodes that can be used on any flow. You have 5 config nodes that can only be used on the RPi... Flow, and 15 on each of the other tabs. This is the "scope" if the nodes - which can be set by the select box at the bottom of the config node's edit dialog.
I am still a bit fuzzy on the whole "background control" aspect of Node-Red.
But your explanation seems to indicate that the "Hidden" part is not what I thought it was... as in remnants of many downloaded and deleted flows from "assisting" others (and I use the term loosely as I have much to learn myself).